How have you measured? I know people do decline cognitively in old age, but 30s is still young. I wonder if people only think they were sharper when younger. I knew much less in my 20s and made tons of mistakes. The mind has a peculiar way of highlighting good memories and downplaying the bad. The person I was in my 20s would not be able to do my current job.
If it were somehow possible to grant my 20s self these advantages, he’d run circles around me. Not just because he’s cognitively faster, but because he’s able to work for longer periods without feeling fatigued and can focus more deeply thanks to having fewer mental background tasks constantly running and not yet feeling the various time-related pressures that come with age.
Giving my teenage self current knowledge would also be interesting. Even though people aren’t cognitively complete yet at that point, at that age I had the ability to get lost in a project on a dime which was was amazing.
Yes, accumulated knowledge and wisdom can make you more successful with age. It doesn't mean your fluid intelligence and ability to learn new things is keeping up.